[R6RS] I/O

[Michael Sperber]

I've committed up-to-date versions of the I/O SRFIs to the repository
at srfi/io.  The changes I made wrt the versions currently on
srfi.schemers.org are:

- various clarifications and spec tightenings requested in the phone
  conference

- "The return values are unspecified." -> "The unspecified value is
  returned."

- #f -> end of file object

- blob -> bytes

- Symbols for buffer modes.

- Enum-sets for file options.

- Predicates for the various position-related procedures.

- removed `newline'

-- 
Cheers =8-} Mike
Friede, Völkerverständigung und überhaupt blabla